Soliton is a unique pulse which can be transmitted over long distance in its characteristic form by compensating the color dispersion effect of optic fiber through non-linear formation. The soliton has been used for ultralong distance transmission regardless of the color dispersion limitation, by employing an optical amplifier in the soliton transmission to compensate the loss caused by optic fiber. However, accumulation of amplified spontaneous emission ("ASE") noise, caused by the optical amplifiers which are distributed at periodic intervals to compensate the loss by optical fiber, could impose a significant limitation on the maximum transmission distance (which is known as "Gorden-Haus effect").
Recently, in order to avoid Gorden-Haus effect, a sliding-frequency guiding filter (hereinafter referred to as "SFG filter") has been suggested in the art (see: L. F. Mollenauer et al., Optics Lett., 17(22);1575-1577(1992)). In this filter, bandpass filters with slightly different resonant frequency are distributed along the optical amplifiers to prevent the ASE noise accumulation, grounded on a fact that: soliton, a non-linear wave, can be guided along the resonant frequency of the filter, while ASE noise cannot be guided. SFG filter can increase the soliton transmission capacity, by reducing the limitation of transmission distance caused by Gordon-Haus effect and by decreasing the interaction of soliton signals. Also, application of wavelength division multiplexing (WDM) system in the soliton control in a frequency domain allows more channels to be used, since the interaction with soliton signals from different channels can be reduced (see: U.S. Pat. No. 5,357,364).
However, the conventional SFG filter, though it can be realized easily by employing simple Fabry-Perot filter, has not been practically applied in the art because hundreds of these filters operating at slightly different resonant frequencies from each other are required in the long distance soliton transmission system. For example, for the transoceanic soliton transmission system applied to a distance of about 10,000 km over the Pacific ocean, approximately 300 SFG filters, under a consideration of an optical amplification distance of 30 to 50 km, are required and each filter should be designed, manufactured and implemented in a separate manner.
Accordingly, there are strong reasons for exploring and developing an improved SFG filter in order to solve the said problems in the art.